Hybrid photovoltaic-thermoelectric solar cells: State of the art and challenges

Concerns about sufficient availability of fossil sources in the near future to satisfy the increasing demand for energy, along with an increasing consciousness about global warming and emission-related climate change, have further pushed research and technology development of the so-called renewable energy sources. Solar energy is by far the most widely used of such sources. Photovoltaic (PV) cells have actually undergone an impressive efficiency enhancement over the last decades, stepping up from ~13% in 1977 (single-junction, nonconcentrated single-crystalline solar cells) to 140the current record efficiency of 44% in concentrated, three-junction lattice-matched solar cells [1]. At the same time, solar energy costs have dropped from 77 USD/W (1977) to 0.30 USD/W (2015) [2]. Currently, PV research is addressing two major avenues, namely that of further enhancing PV efficiency and that of developing cells based on noncritical raw materials. On the former side, spectral matching is pursued. Actually, as will be shown in greater detail in Section 7.4, optimal photon energy conversion is achieved only for photons with energy very close to the PV material energy bandgap E g. Thus, enhanced efficiency may be obtained either by stacking materials with different E g in multijunction cells or by reshaping the solar spectrum so as to increase the spectral fraction that may be more efficiently converted into electric energy by a given set of PV materials. On the latter side, instead, new PV materials have surfaced over the last years, including solid solutions such as the CIGS (CuIn x Ga1-x Se2) [3] and the CZTS (Cu2ZnSnS4-x Se x) family [4].